iPhone 6 went black

Hello, i expierenced strange thing with iPhone 6. Customer came with this phone and said screen is blinking (i assumed it will be broken lcd) i wanted to check it, and it's not working at all. Customer said it was working that same day.

So i tried all the basics to check if this phone is alive:

• power + home button

• disconnecting and reconnecting battery

• itunes restore - yes, phone is recognized by the itunes!

i tried another screen, no luck, i checked if it's backlight problem by lighting bright flash in to the screen, it's absolutely nothig, screen is not showing anything at all.

I had another iphone 6 motherboard laying around, so i connected it and everything works, so i'm assuming problem is there, but how it's recognized by itunes, and completely not showing any image?

any other ideas to try?